A ceramic support widely used for a catalyst is made of cordierite (2MgO.2Al2O3.5SiO2) having low thermal expansion and high thermal shock resistance. This ceramic support is generally used as a catalyst body by shaping cordierite into a honeycomb form and, after coating the surface with γ-alumina, loading a noble metal catalyst thereon. The coated layer is formed because the specific surface area of cordierite is small and a necessary amount of catalytic component cannot be supported on the cordierite as it is.
In this way, conventional ceramic supports are used under the condition that γ-alumina having a large specific surface area is coated thereon. However, the coating of the support surface with γ-alumina disadvantageously incurs an increase in the heat capacity due to an increase of the weight. Particularly, in recent years, studies have been made to decrease the heat capacity by making the cell wall of the honeycomb support thinner so as to attain early activation of the catalyst, but this effect is considerably reduced by the formation of the coated layer. Furthermore, the coated layer gives rise to problems such as the thermal expansion coefficient becomes large or the pressure loss increases due to a reduction in the cell opening area.
Accordingly, various studies are being made regarding a ceramic support where a catalytic component can be loaded without forming a coated layer. For example, the specific surface area of cordierite itself can be increased by a method of subjecting the cordierite to an acid treatment and then to a heat treatment, but this method is disadvantageous and impracticable because the crystal lattice of cordierite is destroyed by the acid treatment or the like and the strength is decreased.
On the other hand, Japanese Unexamined Patent Publication (Kokai) No. 2002-346383 proposed a ceramic support where at least one of the elements constituting the substrate ceramic is replaced with an element other than the constituent elements and thereby a catalytic component can be made to be supported directly on the substrate ceramic. In this ceramic support, a catalytic component can be directly supported by chemical bonding and a coated layer for increasing the specific surface area is not necessary. Moreover, because there is no problem of reduction in the strength accompanying an acid treatment or the like, this ceramic support is promising as a support for use in a catalyst, for automobiles, which is required to have durability.
In the ceramic support of Japanese Unexamined Patent Publication (Kokai) No. 2002-346383, an element having a d or an f orbital in its electron orbital can be used as the element for replacing the constituent element of the substrate ceramic. It is considered that the energy level of the element having a d or an f orbital is close to that of a catalytic component and therefore, donation of an electron readily occurs to facilitate the chemical bonding.
However, as a result of studies by the present inventors, the catalytic performance after a heat-endurance test is revealed to greatly vary depending on the kind and combination of replacing elements. Also, the introduction of a replacing element sometimes causes increase in the thermal expansion coefficient and it is keenly demanded to realize early activation while maintaining the properties of the cordierite.
The present invention has been made under these circumstances. An object of the present invention is to obtain a ceramic support where a catalytic component is directly supported by the replacement of element, and to obtain a high-performance ceramic support capable of realizing early activation of a catalyst due to direct loading while maintaining the excellent properties of the substrate ceramic, and being less susceptible to heat deterioration, by specifying the kind and combination of optimal replacing elements, the replacing amount and the like.